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Wednesday, 6 July 2011

Geotextile

Along the edges of every tile in a LOFAR high-band array, there are long stretches of fabric, known as geotextile. This material serves two purposes: weed suppression and protection against erosion.

Although the array looks pretty solid, there is actually a small gap between the tiles of the HBA. The tiles are 5000mm, but the spacing of them is 5148mm (that is, and 148 millimetre gap). Normally the HBA tiles are deployed on the ground, so getting to the space between them is quite difficult. And, with wind-blown seed, it would be easy for weeds to grow out of control in the gap between the tiles.

For KAIRA, this is not so much of a problem. Firstly, the entire array will be built up 1.5 metres above the ground on the framesets. This means that weeds will struggle to get as high as the tiles themselves (don't forget that vegetation is quite stunted in the Arctic). Secondly, the framesets make it easy to get underneath and deal with the weeds. Still, the fabric will not hurt and it will certainly help suppress weeds around the anchor points.

However, the other reason — protection again erosion — is more important for KAIRA than for other LOFAR sites.

This is because there is a lot of water at the beginning of summer, when warm weather and rain combine with melting snow to create a lot of liquid water. This runs off the tiles and falls down the gaps. However, unlike a conventional LOFAR station, the raised tiles means that this water is falling not 0.5 metres, but 2 metres.

The geotextile is deployed from a large roll that is wheeled out onto the field. It is drawn out and cut to length to match the section being completed. Then, small slits are made, and the fabric is placed over the survey pegs. The edges are nailed into the soil to hold it temporarily while waiting for the framesets to be deployed and the anchor lines drilled in and attached.

Trainees at the KAIRA site lay-out the geotextile.

A second length of fabric goes cross-ways to complete the lattice. This also neatly defines the squares which, although not critical, certainly make life a little easier to work out what goes where and to avoid accidentally kicking the survey pegs.

A completed section of geotextile near the north corner of the KAIRA HBA.